Because of its characteristic shape, the electric resistance wire coil configuration disclosed by the Loguin U.S. Pat. No. 1,171,059, Feb. 8, 1916, is today called a porcupine coil.
As disclosed by the Henriksen U.S. Pat. No. 1,163,536, Dec. 7, 1915, such a porcupine coil, when encased by a tube through which fluid can be flowed, potentially provides a high efficiency electric resistance fluid heater.
To make a porcupine coil, the electric resistance wire is wound on a flat mandrel so as to produce flat convolutions having looped ends which being of small radius can be called peaks. When released from the mandrel, the inherent spring-back of the wire causes the flat convolutions to partially rotate in the same directions so that the released coil automatically becomes a substantially helical series of substantially flat convolutions. These convolutions are bunched together throughout the length of the coil, requiring the coil to be stretched to separate the convolutions and prevent them from short-circuiting.
In the Henriksen patent the coil is held stretched by its ends being anchored to terminals, and in the Loguin patent the coil is suspended vertically by its top end, gravity apparently being relied on to hold the coil convolutions separated. Neither arrangement can provide a stable arrangement if the coil is subjected to high velocity fluid flow.
In the case of electric resistance wire cylindrically coiled with circular convolutions, it is old to hold the convolutions spaced apart by casting fluid or plasticized insulating material around the outside of the coil, which hardens to form a tube around the coil, in the inside of which the coil convolutions are partially embedded. This is exemplified by the Beebe U.S. Pat. No. 786,257, Apr. 4, 1905. This expedient permits only about half of the wire surface area to be exposed to fluid flow through the tube.